Two designs of heat exchanger are presently in general use for reboiler-condensers in cryogenic, refinery and chemical applications. One type of heat exchanger in current use is a vertical shell and tube heat exchanger. To achieve a sufficiently high degree of heat transfer at relatively low temperature differences with this design, enhanced boiling layers (EBL) are used. An EBL typically has a structure comprising a multitude of pores that provide boiling nucleation sites to facilitate boiling. An EBL is applied to the inside of the tubes, and longitudinal flutes are provided on the outside of the tubes to facilitate heat transfer.
Enhanced boiling layers were first proposed for heat exchangers in U.S. Pat. No. 3,384,154. This patent discloses mixing metal powder in a plastic binder in solvent and applying the slurry to a base metal surface. The coated metal is subjected to a reducing atmosphere and heated to a temperature for sufficient time so that the metal particles sinter together and to the base metal surface. U.S. Pat. No. 3,457,990 discloses an enhanced boiling surface with reentrant grooves mechanically or chemically formed therein.
Other methods of applying EBLs have been disclosed. GB 2 034 355 discloses applying an organic foam layer to a metal heat transfer member and plating the foam with metal such as copper first by electroless, then by electrodeposition. U.S. Pat. No. 4,258,783 discloses mechanically forming indentations in a heat transfer surface and then electrodepositing metal on the pitted surface. GB 2 062 207 discloses applying metal particles to a metal base by powder flame spraying. EP 303 493 discloses spraying a mixture of metal and plastic material onto a base metal by flame or plasma spraying. U.S. Pat. No. 4,767,497 and U.S. Pat. No. 4,846,267 disclose heat treating an aluminum alloy plate to produce a precipitate followed by chemically etching away the precipitate to leave a pitted surface. EP 112 782 discloses applying a mixture of brazing alloy and spherical particles to a metallic wall and heating the coated wall to melt the brazing material.
A common heat exchanger used in cryogenic, refinery and chemical applications is the plate-fin brazed aluminum heat exchanger fabricated by disposing corrugated aluminum sheets between aluminum parting sheets or walls to form a plurality of fluid passages. The sheets are either clad with an aluminum brazing layer or a layer of brazing foil is inserted between the surfaces to be bonded. When heated to a predetermined temperature for a predetermined period of time, the brazing foil or cladding melts and forms a metallurgical bond with the adjacent sheets. The resulting heat exchanger contains numerous passages consisting of alternate layers of closely spaced fins. A typical arrangement of alternate layers of passages each containing fins with a density of 6 to 10 fins/cm (15 to 25 fins/inch), and a fin height of 0.5 to 1 cm (0.2 to 0.4 inch). In a common application, a first series of alternating passages carry vapor for condensing, while a second series of alternating passages carry a liquid for boiling. Typical brazed aluminum heat exchangers must be able to withstand 2068 to 2758 kPa (300 to 400 psia).
Patents proposing replacing fins with an enhanced boiling layer in the boiling passages of a brazed heat exchanger include U.S. Pat. No. 5,868,199; U.S. Pat. No. 4,715,431 and U.S. Pat. No. 4,715,433. These patents propose to stack aluminum sheets each with an EBL applied on one side to define boiling channels and with fins on the other side of the aluminum sheets to define condensing channels. Layers of brazing material are disposed between bonding surfaces in the stack, and the stack is subjected to heating over a period of time to obtain a brazed heat exchange core. Such brazed aluminum heat exchangers described in these patents have not been commercialized because EBLs are typically brazed at 565° to 593° C. (1050° to 1100° F.) while the subsequent brazing of the metal components together occur at around 593° to 621° C. (1100° to 1150° F.). Maintaining the integrity and effectiveness of the EBL, particularly the porous structure provided by the mutually bonded metal particles, during the second hotter heat treatment to effect brazing has been difficult. This difficulty accounts for the lack of commercially available brazed heat exchangers with EBL in the boiling passages.
In U.S. Pat. No. 7,677,300, a method for making brazed heat exchangers as well as the heat exchangers were disclosed. However, In fabrication development trials conducted for the high flux brazed aluminum heat exchanger product, it was found that the initial controlled atmosphere brazing (CAB) process to apply the High Flux coating to the AA-3003 aluminum parting sheets results in the presence of some residual brazing flux of the Potassium Aluminum Fluoride (KAlFx) variety throughout the porous metal matrix metallurgically bonded to the aluminum sheet substrate. While this fact in itself is not surprising, it has been discovered that the residual brazing flux, even in relatively small amounts remaining after ultrasonic cleaning of the High Flux-coated sheets and with its relatively low vapor pressure, can have detrimental impact on the subsequent vacuum aluminum brazing (VAB) process that is routinely used for fabrication of BAHX cores using the AA-3003 parting sheets.